A molecular biological study of the expression of murine leukemia virus (MuLV) is proposed. Transcription of MuLV proviral DNA will be studied by pulse labeling of nuclear RNA, and virus-specific nuclear RNA greater than 38S will be studied in particular. Transcription of in vitro recombinant DNA clones carrying M-MuLV proviral DNA will be performed. Cytoplasmic 38S and 24S mRNAs will be analyzed by S1 mapping and nucleic acid sequencing. Particular attention will be focused on the 38S mRNA, since it codes for three distinct polyproteins. Processing and function of viral polyproteins will also be performed. Major emphasis will be on glycosylated gag polyprotein, which is a unique property of murine retroviruses. In order to determine a function, MuLV mutants defective for glycosylated gag polyprotein will be isolated by standard genetic and recombinant DNA techniques. The nature of extracellular gag antigen which binds to extracellular matrices will be investigated. Processing of env polyprotein precursors of two MuLV strains will be compared. It is possible that they differ by cleavage of signal peptide. Molecular cloning of mouse virus-like 30S (VL30) sequences will be performed, and the clones will be studied by DNA sequence analysis to search for polypeptides that may be encoded. The recombinant clones will also allow detailed study of intracellular VL30-specific RNA. These studies will give information on the life cycle of MuLV, and they might identify molecular steps in the pathogenesis by this leukemogenic virus. Additionally, information on host cell mechanisms of gene expression may be obtained.